This invention relates to internal combustion engines of the two-stroke-cycle spark ignition gasoline and diesel types, more specifically, to combustion chamber and displacer piston constructions designed to improve the scavenging of combustion products and the introduction of fresh charge when overhead cam operated inlet and exhaust valves are used. This invention also relates to a selective valve timing method that further improves the engine's scavenging effectiveness and increases its expansion ratio.
The specific output of a two-stroke-cycle engine is largely determined by its scavenging effectiveness which is defined as the ability to successfully remove burned and residual gases of the preceding cycle from the cylinder and fill the cylinder with fresh charge. The scavenging effectiveness itself varies with the blower or scavenging pump delivery ratio and with the particular airflow pattern through the cylinder and in the present invention also through the combustion chamber.
In an attempt to improve the scavenging effectiveness, several two-cycle diesel engines employing cam operated inlet and exhaust valves have been proposed. Many serious deficiencies and drawbacks, however, are associated with these earlier designs. For example, one particular internal combustion engine, as described in U.S. Pat. No. 2,222,134, employs an inlet valve alongside of the cylinder and a relatively flat, laterally extending combustion chamber in an attempt to provide a more thorough scavenging of combustion products by allegedly directing the incoming air into a single axial flow column which flows into the cylinder, hits the piston, then flows upward until it exits through an exhaust passage. The major difficulty with the configuration of this particular combustion chamber is that a small flow area produces high velocity air that enters the chamber, travels horizontally through the chamber and then makes a 90.degree. turn and flows vertically into the cylinder. Instead of directing a strong, single column, axial flow of air downward into the cylinder, the airflow short-circuits and scatters across the cylinder, reducing the overall effectiveness of the scavenging process. Another major difficulty with the above engine is the narrowness of the opening between the combustion chamber and the cylinder. The diameter of the opening is such that the air velocity would be much too high for modern engines, causing turbulence within the cylinder, channeling, and high pumping losses. Moreover, the narrow opening increases the time required for complete scavenging, thus reducing engine RPM's.
Another attempt to improve the efficiency of a two-cycle diesel engine is depicted in FIG. 6 of U.S. Pat. No. 2,136,070, but this design also fails to direct a single column of air downward into the cylinder or even to specify how scavenging is achieved. This lack of a unidirectional airflow produces poor and limited scavenging and the new charge is likely to short-circuit.
Therefore, it is a major object of this invention to overcome the limitations of the prior art and improve the two-stroke cycle operation by employing overhead valves in combination with an improved combustion chamber in order to provide a flow of scavenging air that is more effective than the prior art. Also, the improved combustion chamber is oriented to provide high turbulence therein during firing in order to promote combustion.
Another object is to provide an improved displacer piston that can cooperate with the combustion chamber to further increase the amount of turbulence created therein as well as to adjust the clearance volume and improve the geometry of the chamber.
Yet another object is to further improve the scavenging efficiency by providing unsymmetrical valve timing for the operation of the inlet and exhaust valves.
A further object is to improve the engine thermodynamic cycle by increasing the expansion ratio and reducing the compression ratio by utilizing asymmetrical valve timing in conjunction with unsymmetrical valve timing. Asymmetrical timing for the purpose of this invention is defined as the skewing of the engine timing about the bottom dead center position of the piston.
A further object is to provide an engine of high specific output that is compact in size and reduced in weight.
A still further object is to provide a two-cycle engine of simpler design requiring fewer cylinders for the same power and uniform torque.